This disclosure relates generally to X-ray systems and methods, and more particularly to a system and method of determining the exposed field of view in an X-ray radiograph.
This invention generally relates to computer aided detection and diagnosis (CAD) of an image set. More particularly, this invention relates to a method and system for computer aided detection and diagnosis of dual energy (“DE”) or multiple energy images, as well as of radiographic images, computed tomography images, and tomosynthesis images.
The classic radiograph or “X-ray” image is obtained by situating the object to be imaged between an X-ray emitter and an X-ray detector made of photographic film. Emitted X-rays pass through the object to expose the film, and the degree of exposure at the various points on the film are largely determined by the density of the object along the path of the X-rays.
It is now common to utilize solid-state digital X-ray detectors (e.g., an array of switching elements and photo-sensitive elements such as photodiodes) in place of film detectors. The charges generated by the X-rays on the various points of the detector are read and processed to generate a digital image of the object in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the imaged object, and so on.
Dual energy (DE) imaging in digital X-Ray combines information from two sequential exposures at different energy levels, one with a high energy spectrum and the other with a low energy spectrum. With a digital X-ray detector, these two images are acquired sequentially and processed to get two additional images, each representative of attenuation of a given tissue type, for example bone and soft tissue images. A multiple energy imaging system can be built that can be used to further decompose the tissues in an anatomy. A series of images at different energies/kVps (Energy 1, . . . Energy n) can be acquired in a rapid sequence and decomposed into different tissue types (Tissue 1, . . . Tissue n).
Computed tomography (CT) systems typically include an x-ray source collimated to form a fan beam directed through an object to be imaged and received by an x-ray detector array. The x-ray source, the fan beam and detector array are orientated to lie within the x-y plane of a Cartesian coordinate system, termed the “imaging plane”. The x-ray source and detector array may be rotated together on a gantry within the imaging plane, around the imaged object, and hence around the z-axis of the Cartesian coordinate system.
The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a ray path projected from the x-ray source to that particular detector element. At each gantry angle a projection is acquired comprised of intensity signals from each of the detector elements. The gantry is then rotated to a new gantry angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set. Each acquired tomographic projection set may be stored in numerical form for later computer processing to reconstruct a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT tube, flat-panel thin-film-transistor array, or may be converted to a film record by means of a computer-controlled camera.
The fan beam may be filtered to concentrate the energies of the x-ray radiation into high and low energies. Thus, two projection sets may be acquired, one at high x-ray energy, and one at low x-ray energy, at each gantry angle. These pairs of projection sets may be taken at each gantry angle, alternating between high and low x-ray energy, such that patient movement creates minimal problems. Alternatively, each projection set may be acquired in separate cycles of gantry rotation, such that x-ray tube voltage and filtering need not be constantly switched back and forth.
Diagnosis from radiographic images, computed tomography images, and other medical images has traditionally been a visual task. Due to the subjective nature of the task, the diagnosis is subject to reader variability. In addition, due to the underlying and overlying structures relevant to the pathologies of interest, visual assessment can be difficult. Subtle rib fractures, calcifications, and metastatic bone lesions (metastases) in the chest can be difficult to detect on a standard chest X-ray. As an additional example, only 5-25% of pulmonary nodules are detected today with chest radiographs, but 35-50% are visible in retrospect. In a CT acquisition, different regions of the imaged object can be composed of different tissues of differing densities such that the total attenuation (thus CT number and pixel value) are the same. These two regions would have identical representation in the image, and thus be indistinguishable. Dual energy CT offers the ability to discriminate between the two tissue types. Traditionally, this discrimination would still be a visual task.